Two protesters have been killed in clashes with police in the Ukrainian capital Kiev.

Prosecutors confirmed they had died from bullet wounds. They are the first fatalities since anti-government protests began in November.

Wednesday's clashes began after police moved in to dismantle a protest camp.

After talks with President Viktor Yanukovych, one opposition leader, Vitali Klitschko, threatened to lead protesters "on the attack".

"Today they [the police] are preparing to clear us out of the Maidan (Independence Square)," Mr Klitschko declared.

"We must do all we can to stop them clearing us out."

He said the president could end the stand-off by calling early elections but that "tomorrow, if the president does not respond... then we will go on the attack", to roars of approval from the crowd.

Meanwhile, the US embassy in Ukraine said it had revoked the visas of "several Ukrainians who were linked to the violence". It did not give names, but said it was "considering further action against those responsible for the current violence".

The clashes took place on the day that new anti-protest laws come into force. Parliament approved the laws last week, triggering renewed protests which spilled into violence on Sunday night.

Hundreds of people have been injured, though some of the violence has been blamed on a little-known far-right group, Right Sector.

Analysis

By David SternBBC News, Kiev

Although numerous details surrounding the protesters' deaths are still unclear - including whether it was indeed a police rifle that killed one activist - the simple fact that there have been losses of life will undoubtedly drive the anti-government movement to new, and possibly explosive, levels of outrage.

The ongoing fighting between demonstrators and riot police has been ferocious, but limited; hostilities have been restricted to a small corner of the capital near the government building, and the two sides observe many breaks in clashes.

But all this may be about to change. More activists are flooding into Kiev from the west, and crowds are massing at the site of the clashes. As the anger and the number of people mounts, the likelihood of a massive outbreak of violence, from either side, increases.

The tension is also rising in Kiev's Independence Square. Activists are forming self-defence groups and reinforcing the barricades. After PM Azarov announced that authorities could use force to restore order, protesters are preparing themselves for any possible development.

The anti-government movement started in protest at Mr Yanukovych's decision in late November to pull out of a landmark treaty with the EU, but has expanded to demand his resignation.

Fireworks

Wednesday's violence began in a small area around Hrushevskyy Street, a road leading to government buildings and also close to the main protest encampment at Maidan (or Independence Square).

Many of the protesters have been wearing helmets and facemasks in defiance of the new laws that ban the wearing of such headgear at protests.

The laws also prescribe jail terms for anyone blockading public buildings and outlaw unauthorised tents in public areas.

European Union leaders have expressed shock at the deaths and called for on all sides to halt the violence.

"If there is a systematic violation of human rights, including shooting at peaceful demonstrators or serious attacks to the basic freedoms, then we have to rethink our relationship with Ukraine," said Jose Manuel Barroso, the president of the EU Commission.

The US also strongly condemned the escalating situation.

"Increased tensions in Ukraine are a direct consequence of the Ukrainian government's failure to engage in real dialogue and the passage of anti-democratic legislation," a US state department spokeswoman said.

Russia has accused the EU and US of "outside interference" in Ukrainian affairs.